The growth of global demand of energy, and the necessity to master greenhouse emissions, may lead to the introduction of a new and universal energy carrier, hydrogen. Today most of energy production comes from hydrocarbons: oil (18%), coal (30%) and natural gas (48%), and only about 4% comes from water by electrolysis. In a long term the prospected lack of fossil resources leaves only water and maybe renewable biomass as the sustainable candidate for hydrogen production.
Research on thermochemical water-splitting cycles began about 40 years ago, and since that time hundreds of technical articles about them have been published. More than 200 thermochemical cycles have been reported, and a number of them have been evaluated quantitatively by computer model simulations. A search program, which uses only the free energies of formation has been developed to find new thermochemical cycles.
One of the major efforts in the development of these cycles was done at the Joint Research Centre at Ispra (Varese, Italy). The program was approved by the Council of Ministers of the European Communities and covered the period 1973-1983. In another ten-year program in the USA (Gas Research Institute), a very simplified evaluation of 200 distinct thermochemical cycles was made, and finally eight cycles were operated successfully with recycled materials to achieve proof-to-principle.
The most promising way to produce hydrogen without producing CO2 is splitting water by high temperature energy from the sun, nuclear sources, or waste heat in thermochemical cycles. There water is decomposed into hydrogen and oxygen via a chemistry using intermediate substances, which are cycled, and the energy needed is introduced as heat. About 100 thermochemical cycles have been found. Four of them were assessed for further development: first the hybrid sulphur cycle—‘the Westinghouse cycle’, and then its three close challengers, the Ispra Mark 13 hybrid cycle, the UT-3 cycle, and the sulphur-iodine (S—I) cycle.
Except for the UT-3 cycle, the main reaction in the most promising thermochemical processes is the decomposition of sulphuric acid (1), which is endothermic and takes place at high temperature.H2SO4=H2O+SO2+½O2  (1)
This reaction is complemented by other reactions, which then close the thermochemical cycle in its variants as:
Westinghouse Electric Corporation pursued the development of a closure, which is called a hybrid sulphur cycle (HyS process) because one of the reactions (2) is electrochemical.2H2O+SO2+(elec)=H2SO4H2  (2)The hybrid sulphur cycle is described for example in U.S. Pat. No. 4,412,895.
Generals Atomics pursued the development of a sulphur-iodine cycle, and determined the conditions under which the products of the Bunsen reaction of water, iodine and sulphur dioxide form two phases, one rich in HI and the other rich in H2SO4. The cycle, consisting of the Bunsen reaction (3) and the decomposition reaction (4), is also known today as the “GA process”. The process is described for example in U.S. Pat. No. 4,089,940.2H2O+SO2+I2=H2SO4+2HI  (3)2HI=I2+H2  (4)
The Ispra Mark 13 is a hybrid cycle and a complete bench-scale continuous process was built and operated at Ispra. The reactions in this closure are:2H2O+SO2+Br2=H2SO4+2HBr  (5)2HBr+(elec)=Br2+H2  (6)
The UT-3 process, being invented at the University of Tokyo in the 1970s, and selected by JAERI (Japan Atomic Energy Research Institute, Ibaraki, Japan) for a further development, consists of four gas-solid reactions: two Ca-compounds reactions (7), (8) and two Fe-compounds reactions (9), (10). This process is operated in a cyclic manner in which the solids remain in their reaction vessels and the flow of gases is switched when the desired extent of reaction is reached.CaBr2+H2O=CaO+2HBr  (7)CaO+Br2=CaBr2+½O2  (8)Fe3O4+8HBr=3FeBr2+4H2O+Br2  (9)3FeBr2+4H2O=Fe3O4+6HBr+H2  (10)
From the four thermochemical cycles above, the Ispra Mark 13 cycle has not been studied anymore in the recent years, and the same concerns the adiabatic UT-3 cycle, which is no longer the focus of JAERI's investigations in this area, either. So, recently the S—I and HyS cycles are favoured in the world in comparison with other known thermochemical cycles studied over the last 35 years.
The most energy demanding part of the thermochemical cycles is the splitting of H2SO4 (to SO2+H2O+½O2), which forms during the cycle. The original invention, as well as further developments of the S—I cycle, suggest the application of nuclear energy as the primary heat source, and this is the case concerning the HyS process, too.
Efficiencies (thermal to hydrogen) in the range of η=47-56% have been calculated for the full process of the sulphur-iodine cycle, and it has been shown that thermochemical cycles have potential to deliver overall system efficiencies in excess of 40%. This is much lower than the efficiency an electrolysis producing hydrogen by water splitting. The efficiency of electric power conversion (electricity to hydrogen) is currently about 80%.
The sulphur-iodine (S—I) cycle can be split into the following reactions (11)-(17), in which the temperatures between brackets are approximate and depend upon the pressure which is not necessarily uniform in the different parts of the cycle. The practical stoichiometrics are:(9I2)I+(SO2)g+(16H2O)I→(2HI+10H2O+8I2)I+(H2SO4+4H2O)I [120° C.]  (11)L2:(2HI+10H2O+8I2)I→(2HI)g+(10H2O+8I2)I [230° C.]  (12)(2HI)g→H2+(I2)I [330° C.]  (13)L1: (H2SO4+4H2O)l→(H2SO4)l+(4H2O)g[300° C.]  (14)(H2SO4)l→(H2SO4)g[360° C.]  (15)(H2SO4)g→(SO3)g+(H2O)g[400° C.]  (16)(SO3)g→(SO2)g+½O2[870° C.]  (17)
Reaction (11), named the Bunsen reaction, forms the first section. It proceeds exothermically in the liquid phase and produces two immiscible aqueous acid phases whose compositions are indicated between brackets: L1 phase which is aqueous sulphuric acid and L2 phase, named HIx, which is a mixture of hydrogen iodide, iodine and water. The Bunsen reaction, as it has been written in (11), involves an excess of both water and iodine, with the reference to stoichiometric amounts. The excess of water is required to make the reaction spontaneous, and the excess of iodine induces the phase separation, which is a key point of the process. Such excesses, however, are quite unfavourable for the following HIx section as well as for the energy balance.
Reactions (14)-(17) belong to the second section. Reactions (15)-(17) proceed in the gas phase and produce H2O, SO2 and O2. These gases are cooled down prior to being bubbled in the Bunsen reactor for separating oxygen from SO2 and H2O. Alternatively, oxygen can be separated from the gas before it enters the Bunsen reactor. HI decomposition according to (13) must be achieved from the HIx mixture produced in the previous Bunsen reaction. This acid section appears to be the best known step of the cycle, because of the experience gained in the sulphuric acid industry. Sulphuric acid is concentrated through a series of flashes starting from low pressure. It is then dehydrated, before SO3 is decomposed into SO2. This decomposition being only partial, undecomposed SO3 is recombined with water, which allows to recover its heat content.
The third section is formed by reactions (12) and (13). In reaction (12), HI is separated from L2. This separation is the most critical stage of the cycle. Reaction (13) is the thermal decomposition of HI. It has also been proposed to perform reactions (12) and (13) in the same reactive distillation column.
The hybrid sulphur process (HyS, the Westinghouse process) is an all-fluids, two-step thermochemical cycle, involving hydrogen production electrolytically, and decomposition of sulphuric acid as another process step. The net result of the two reactions is the decomposition of water into its constituents, hydrogen and oxygen.
The system has three main processing units:                a SO2 depolarized electrolysis tank for producing gaseous hydrogen and a water-sulphuric acid mixture,        a sulphuric acid concentration and decomposition step, and        separation of O2 from SO2 before introducing SO2 back into the electrolysis tank.The first step of the HyS process involves hydrogen production in an electrochemical cell by reaction (18). Sulphur dioxide is oxidized on the anode of an electrochemical cell, while protons are reduced on the cathode is to produce hydrogen. The electrolyte used in the cell is sulphuric acid, and sulphur dioxide is used to scavenge the anode.SO2+2H2O→H2SO4+H2[electrochemical, 80-120° C.]  (18)The theoretical equilibrium voltage required to decompose water under standard conditions (25° C., infinite dilution) is E0H2/H2O=1.23 V, and the commercial direct water electrolyzers, which have to deal with system efficiency and overvoltage, operate with 1.8-2.6 V per cell. The presence of sulphur dioxide depolarizes the anode and reduces the reversible voltage relative to that required for the direct dissociation of water.        
The acid decomposition step involves multiple process operations, including preheating, acid concentration, acid vaporization, acid dissociation, and sulphur trioxide decomposition as is presented earlier by reactions (15)-(17). This process step is common to all sulphur-based thermochemical cycles, so the results obtained in the development of this section of the sulphur-iodine process can be directly applied to the HyS cycle, too.
After the sulphuric acid decomposition unit, the oxygen thermally decomposed from SO3 is removed from the stream. The separated sulphur dioxide is recycled to the electrolysis tank, and oxygen is either used in some other process or vented.
The original thermochemical cycles described above have been developed to with the application of nuclear energy in mind and the application of SO2 without recycling and decomposition of sulphuric acid has not been discussed in this connection.
In the prior art there is also described in JP 2005219033 a method, which removes sulphur oxide from a gas and utilizes it for hydrogen and sulphuric acid production. Sulphur-oxide containing gas is discharged from a furnace such as a coal/oil burning boiler, metal refining furnace and sulphur furnace. Sulphur oxide containing gas is contacted with bromine and water, which results forming of sulphuric acid and a gas containing hydrogen bromide (HBr) and water. Iron bromide (FeBr2) and water are reacted to form ferric oxide (Fe3O4) hydrogen bromide and hydrogen. Ferric oxide and hydrogen bromide are reacted to generate bromine and iron bromide. Bromine is used to contact sulphur oxide gas and iron bromide to form hydrogen. The method seems to be a modification of the UT-3 process.
JP 8071365 relates to a method to use an oxidation/reduction system in the desulphurization of sulphur oxide containing exhaust gas for obtaining sulphuric acid and hydrogen as byproducts. Exhaust gas is contacted with a sulphur dioxide absorbing solution containing dissolved iodine in water. The solution forms two layers the light phase containing sulphuric acid and the heavy phase hydrogen iodide. Hydrogen iodide is electrolyzed to produce hydrogen and iodine. Separated iodine is reused for absorbing sulphur dioxide gas. The light phase containing sulphuric acid is concentrated. The method is a modification of SI process without sulphuric acid decomposition.
Both cited Japanese methods produce sulphuric acid in addition to hydrogen. However, each mole of sulphuric acid in the light phase (H2SO4/H2O) is initially accompanied with 5 moles of water, which means that the acid is dilute acid, which is not a commercial product. There is said in JP 8071365 that the sulphuric acid will be concentrated but not said how it takes place. As is known, concentration by water evaporation does not succeed above 60 to mol-% (≈90 wt %) and for a commercial product (100 wt %) the acid has to be concentrated in a sulphuric acid plant. So the sulphuric acid production with said method is not possible without connection to sulphuric acid plant.